As you must tune your DVB device to the right frequency to be able to receive something, you will want to know which frequencies are in use at your place of residence. Note that the frequencies used for North America, Europe, Japan, etc. etc. all differ from one another. Historically, the frequencies used under the older analog systems differed, and when countries began to make the transition to digial systems, most have opted to continue to use their old channel frequency schemes so as to make the switch from analog to digital systems as seemless as possible for the end user.
The handiest tool for scanning frequencies is "Scan", which is a utility included within the LinuxTV dvb-apps package. Scan is especially handy as it uses pre-configured sender lists for many locations, then tunes into the frequencies and lists the names for all available PIDs.
In addition, there are a number of Channel Frequency lists for various countries/regions/locations/service_providers within the wiki that have been submitted by users.
For some other general sources (which may or may not be that useful) for finding local channel frequency lists, see: